/Engine/Plugins/Importers/USDImporter/Source/USDStage/Public/USDStageActor.h
Loads the stage with RootLayer if its not loaded already, and returns the either the isolated stage (if any) or the base stage
[UE::FUsdStage](API\Plugins\UnrealUSDWrapper\UsdWrappers\FUsdStage) & GetOrLoadUsdStage()